Method of providing adaptive echo cancellation in transmission of digital information in duplex, and apparatus for performing the method

ABSTRACT

A method and an apparatus, in a telecommunication system for transmission of digital information in duplex over a signal conductor pair for automatically adjusting a balance filter included in a hybrid coupler circuit with the aid of adaptive echo cancellation so that the local data transmitter does not disturb the local data reception, without utilizing any particular testing procedure in initiating the adaptive echo cancellation. A control signal (ε k ) is formed, with the help of which the correction unit (KB) is caused to generate signals for rapid updating of the balance filter (B) parameters, in spite of that knowledge is lacking in the initiation instant concerning the value of the signal transmitted from the remote end to the apparatus, and that no counteraction between level correction and filter adaption is obtained. The apparatus includes a quantizer (Q) in which a sampled, received signal (r k ) is quantized, an estimated signal (a k ) of data sent from the remote end being obtained. Said estimated signal is multiplied by a reference signal from a reference unit (V), the result being subtracted from the received signal (r k ) so that an error signal (e.sub. k) is formed. The sign of the error signal is correlated with the estimated signal (a k ) in a correction unit (KV), which calculates a correction (ΔV) which is added to the reference signal in order to correct it. The sign of the error signal (e k ) is also added to the estimated signal (a k ) thereby forming said control signal (ε k ) which is correlated with the data vector (b k ) in the correction unit (KB) which calculates a correction value (Δc) which is supplied to the input of the balance filter for correction of the filter parameters.

TECHNICAL FIELD

The present invention relates to a method of providing adaptive echocancellation in transmission of digital information in duplex over asingle pair of conductors. The invention also relates to apparatus forperforming the method.

BACKGROUND ART

Adaptive echo cancellation is provided in telephony and datacommunication engineering to prevent echo signals from affecting thereception. For example, in a data modem connected to a two-wire line,the transmitter in one of the two-wire directions and the receiver inthe other are connected to the two-wire line via a hybrid coupler, data(b_(k)) being sent from the transmitter across the line to a modem atthe remote end, while conversely, data (a_(k)) is transmitted from theremote end across the line to the local modem receiver. Due todeficiencies in the hybrid coupler, it is unavoidable that a certainproportion of data flow (b_(k)) from the transmitter passes through thehybrid coupler and into the receive path, reception of the data flow(a_(k)) thus being disturbed. Furthermore, disturbing signals occur inthe form of echoes from the local transmission at the remote end, butthe leak signals through the coupler are the ones which dominate andwhich most heavily affect the reception. These leak signals coming fromthe local data flow (b_(k)) and occurring in the local modem detector asdisturbance signals are usually called echo signals, even if they havenot been transmitted across the line, reflected and transmitted backagain to the local modem.

In order to reduce the effect of such echo signals it is known in theprior art to provide a digital-type balance filter, usually a finiteimpulse response (FIR) filter connected to the transmit and receivechannels. The task of the balancing filter is accordingly to form asignal from the transmitting data flow, this signal being subtractedfrom the one which occurs at the detector input, after having passedthrough the hybrid coupler, and which contains leak signals from thetransmit channel. For rapidly adjusting the balance filter parameters,i.e. provide rapid convergence in the balance filter, it is howeverrequired that the correlation between the incoming analogue signal inthe receive path (denoted below w(t)+h(t)) and the transmitted data flowb_(k) be great. The presence of the remote signal w(t) coming from thedata flow (a_(k)) decreases this correlation however, and convergence isslow.

Rapid convergence of the balance filter thus requires that the analogueremote signal w(t) be eliminated in some way when calculating theparameter adjustment. A known method of eliminating or cancelling signalw(t) is to ensure that the remote end transmitter is disconnected duringa test period when the adjustment of the balance filter can be carriedout. Transmission quality will then be entirely dependent on howsuccessful the adaption has been during the test period. The test periodmust be made relatively long. It would be more effective if signal w(t)could be subtracted from incoming signal w(t)+h(t). In digitaltransmission signal w(t) only attains a limited number of amplitudevalues at the sampling instants (if intersymbol interference isneglected). An estimation w(k) of signal w(t) can therefore be made inthe sampling instants k with the aid of a quantizer.

Since the cable attenuation is normally not known, some form ofautomatic level adjustment must be made for obtaining a good estimate.Different systems with adaptive balance filters and adaptive leveladjustments have been described, e.g. in "Adaptive EchoCancellations/AGC Structures for Two Wire, Full Duplex DataTransmission", Bell System Techn. Journal 58 No. 7 (September, 1979),the level adjustments utilize correlation of the received signal with alevel estimation a_(k) of the data sequence transmitted from the remoteend. The methods operate if this estimation is more or less correct. Theleak signal h(t) is however much larger than signal w(t) in manypractical cases. When the balance signal r(t) passes the heavilynon-linear quantizer incorporated in the receiver, all the informationconcerning signal w(t) may be lost if the balance filter is notcorrectly adjusted. In such cases a_(k) will be more heavily correlatedto data flow b_(k) than to data flow a_(k). This has the practicalresult that the adaption of the balance filter and adaption of the levelcounteract each other and convergence cannot be obtained.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a method for adjustinga digital-type balance filter, e.g. one incorporated in a data modemwith rapid convergence, without special test periods and without needingto resort to adaptive level adjustments.

In summary, the inventive method contemplates forming, from the dataflow including both remote signal w(t) and echo signal h(t) coming intothe modem, correction signals to the balance filter only when the levelof the incoming flow exceeds a given reference value, so that the filteris only corrected when the incoming signal level exceeds the referencevalue. This contemplates disconnecting the remote signal, and rapidconvergence (apart from an acceptable residue error) of the balancefilter is enabled.

The method in accordance with the invention is characterized as will beseen from the characterizing portion of claim 1.

DESCRIPTION OF DRAWINGS

The invention will now be described with reference to the appendeddrawings, where FIG. 1 is a block diagram of a data modem utilizing themethod in accordance with the invention,

FIG. 2 is a time diagram of a remote signal,

FIG. 3 depicts in a time diagram examples of a binary data flowtransmitted from the remote end, corresponding to a biphase coded signaland an echo signal to the modem receive side,

FIG. 4 depicts in a diagram sampled values from the echo signalmentioned,

FIG. 5 is a diagram of sampled values ±1 from the remote signal onreception,

FIGS. 6-7 are diagrams of sampled values from the echo signal and remotesignal in comparison with a variable reference level,

FIG. 8 is a block diagram over a preferred embodiment of an apparatus inaccordance with the invention.

BEST MODES FOR CARRYING OUT THE INVENTION

The principle of the inventive method is illustrated in FIG. 1. A datasequence b_(k), comprising ones alternating with zeroes is sent from anunillustrated data source, e.g. a telephone exchange, to a transmitterunit S including a code converter for conversion of the binary flowb_(k), e.g. to biphase coded (analogue) signals s(t), see FIG. 3. Thetransmitter unit S is connected to a hybrid coupler G of known kind,which feeds the biphase coded signal s(t) out onto a two-wire line L.The apparatus illustrated in FIG. 1 may be a part of a modem for datatransmission, the outgoing data being the binary flow b_(k). The datasequence coming into the modem from the remote end of the line L isdenoted a_(k), and is transmitted from the remote end across the line Lsimultaneously as the data sequence b_(k) is transmitted from thetransmitter unit S across the line L to the remote end. The flow a_(k)from the remote end occurs in this assumed case as an analogue signalw(t) (biphase coded) across the input terminal of the hybrid coupled Gfrom the line L. In passing through the coupler G there is added an echosignal h(t) coming from the biphase coded signal s(t) from thetransmitter S. The signals s(t) and h(t) are thus heavily correlated.Across the input of a summing circuit A1 there is thus obtained a signalw(t)+h(t) after filtering in the lowpass filter LP. For inhibiting theeffect of the echo signal h(t), a balance filter B, suitably a FIRfilter, is connected across the input to the transmitter unit S and to aminus input of the summing circuit A1. The balance filter B sends asignal-y(t) to the summing circuit A1, intended to compensate the echosignal h(t), i.e. r(t)=y(t)+w(t)+h(t)≈w(t). The signal r(t) is appliedto the sampling circuit SH, for sampling the signal r(t) in selectedsampling instants t_(k) so that the signal r(k)=r(t_(k)) is formed. Thedata flow b_(k) together with the corresponding biphase coded signals(t) after the transmitter S and the leak signal h(t) are shown in FIG.3.

The signal h(t) derived from the signal s(t) from the transmitters isheavily correlated to this signal and, at least at the beginning oftransmission, is very much stronger at the input to the lowpass filterLP than the remote signal w(t). The balance filter B should thus send asignal y(t) which is also heavily correlated to the signal s(t).

After sampling the signal r(t) in the sampling circuit SH, the valuesr(k) (k=1,2 . . . N) are obtained at the instants t₁, t₂, . . .according to the diagram in FIG. 3. For a correctly adjusted balancefilter B, the input signals r(k) to the detector or quantizer Q will beindependent of the transmitted data sequence b_(k) at the samplinginstants t_(k), i.e. y(t_(k)).

The detector Q is a quantizing circuit which, for the sake ofsimplicity, should be considered here as comprising a comparatordeciding whether the sampling values r(k) are greater or less than zero.The quantity a_(k) across the output of the quantizer Q is +1 if r(k)>0or -1 if r(k)>0, and gives an estimation of the data a_(k) sent from theremote end.

The block V is a reference unit in which a reference value is stored indigital form and may include a digital-to-analogue converter. The outputof the unit is connected to one input of a multiplier M, the other inputof which is connected to the output of the quantizer Q. One input of asecond summing circuit A2 is connected to the output of the multiplier,and the other input of the second summing circuit is connected to theinput of the quantizer Q. The output of the summing circuit A2 isconnected to a sign-forming circuit SG. The output of this is connectedto a third summing circuit A3 and to one input of a correction unit KVfor correcting the reference value stored in the reference unit V. Thecorrection unit KV is connected by its second input to the output of thequantizer Q and may be a multiplier, for example. A third input to thecorrection unit KV connected to the reference unit V is supplied withthe reference value from this unit and the new, possibly corrected valueis supplied to the reference unit V via the output of the correctionunit KV whose output is connected to the reference unit V. The referencelevels in the reference unit V may possibly control the levels in thequantizer Q as indicated by the dashed connection.

An estimated value a_(k) (1 or -1) of data a_(k) transmitted from theremote end thus occurs across the output of the quantizer Q. Aftermultiplying with a reference value v(k) in the multiplier M, there isobtained on the output of the summing circuit A2 a value e_(k)=r(k)-a_(k) ·v(k). In the case where w(t) is a biphase-modulated signal,a_(k) =±1, and two values of e_(k) can be obtained thus:

    e.sub.k =r(k) -v(k) (a.sub.k =+1)

or

    e.sub.k =r(k)+v(k) (a.sub.k =-1)

The sign of e_(k) is obtained across the output of the circuit SG, andit may thus be ±1. The sign of the error signal e_(k) is correlated withthe value a_(k) in the correction unit KV, which computes a correctionfor adding to v(k). The sign of e_(k) obtained across the output of theunit SG denotes whether the value a_(k) ·v(k) was greater or less thanthe sampled value r(k). The value r(k) in turn denotes how near thereceived signal r(t) the received signal w(t) comes, after having passedthe hybrid coupler (addition of h(t)) and after the subtraction of thesignal y(t) from the balance filter. The sign of e_(k) is summed in thesumming circuit A3 to a_(k), and is supplied via a first input to asecond correction unit KB for the balance filter B. There is furthermoresupplied to this unit, via a second and a third input, the referencelevel v(k) and the balance filter parameters cj(k). The correction unitKB then calculates the new parameters cj(k+1 ) of the filter B, whichoccur on the output connected to a control input of the filter B.Correlation of the quantity ε_(k) =sign e_(k) +a_(k) with the data flowb_(k) is performed in the correction unit KB (a correlator), therelationship between the estimated value of the remote signal w(t)represented by sign e_(k) +a_(k) and the transmitted data flow b_(k) isestablished. If the correction unit KB finds that the quantities ε_(k)and b_(k) are correlated, i.e. that there is an undesirable relationshipbetween received data a_(k) and data flow b_(k), the parameters c_(j) ofthe balance filter will be corrected so that this correlation decreases.On the other hand, if the magnitudes ε_(k) and b_(k) are uncorrelatedthere is no relationship between a_(k) and b_(k). The balance filter maythen be considered converged and the corrections will not change theadjustment of the parameters c_(j).

FIG. 2 illustrates the relationship between the sampled received signalr(k), the remote signal w(t), the reference level v(k) and the errorsignal e_(k). At the sampling instants t₁ it is assumed in the one casethat r(k)=r(k)_(I) and a_(k) =+1, e_(k) will then be greater than zeroand the sign of e_(k) is +1. In the other case when r(k)=r(k)_(II)<v(k), e_(k) is less than zero and the sign of e_(k) is -1. In acorresponding manner it is found that for a_(k) =-1, e_(k+1) is lessthan zero when r(k+1)=r(k+1)_(I) and e_(k+1) is greater than zero whenr(k+1)=r(k+1)_(II). As will be described below, there is correction ofthe balance filter B when e_(k) is greater than zero and when e_(k+1) isless than zero.

As mentioned above, the balance filter B is of the FIR type, i.e. theoutput signal in every instant is solely dependent on a limited number Nsamples of the input signal, i.e. the data sequence b_(k). The filterfunction is determined by its parameters c₁ . . . c_(M) and Nconsecutive input signal values b_(k), b_(k-1) . . . b_(k-N+1) the(digital) output signal ##EQU1## and y(t) is the corresponding analoguesignal.

As described in conjunction with FIG. 8, the balance filter may also bea so-called memory filter in which the coefficients c_(j) are stored andare pointed out by an address j=a(b_(k)). The digital output signaly_(k) will then be the coefficient c_(j) (k).

The method in accordance with the invention will now be described indetail with reference to the FIGS. 4-7, illustrating the case of atransmitted data flow b_(k) from the transmitter S which consists ofalternating +1's and where solely one coefficient c₁ in the filter B isupdated. If b_(k=) 1 it is assumed h(t) according to FIG. 4 takes on thevalue b=+1 simultaneously as it is assumed that the value of the remotedata flow w(t)=+1. The correction y(t) is assumed=0. Thus r(k) willbe=w(k)+h(k)=a+b=2, according to FIG. 6 (a=1, b=1), a given levelv(k)=V_(o) being the value of r(k). In such a case a_(k) =1, a_(k)·v(k)=V_(o), e_(k) =r_(k) -V₀ >0, the sign e_(k) =+1 and ε_(k) =1+1=+2.Since ε_(k) and b_(k) are both positive, a positive correction of thecoefficient will be calculated. Simultaneously the correction unit KVindicates that the error of the reference level is positive, and thereference level v(k) must be increased. When the coefficient c₁ is nowincreased to c₂ simultaneously as the reference level v(k) is increased,the value of r(k)=w(k)+h(k)-y(k) will come closer to the new levelv(k+1) according to FIG. 7 (b=1, a=1) and the balance filter will beginto converge. If it is assumed that the data flow a_(k) and b_(k) aresuch that a=-1 (a represents remote data) and b=+1 (represents the leaksignal h(t)), then according to FIG. 6, r(k) will take on a value lessthan v(k)=V₀. In such a case: e_(k) =r(k)-V_(o) <0, sign e_(k) =-1 andε_(k) =-1+1=0. The fact that the error signal is less than zero and thatε_(k) =0 implies that the reference level shall be increased but thatthe coefficient c shall be retained unchanged. For the cases a=1, anda=-1, b=-1 there is respectively obtained that the reference level shallbe decreased with the coefficient c₁ retained unchanged and that thereference level shall be increased with the coefficient c₁ increased. Byway of summary, the situation with the proposed method is that if theabsolute value of the signal values r(k) is greater than a givenreference level v(k), both reference level shall be increased and thecoefficient c in the balance filter changed, whereas if |r(k)| is lessthan v(k) only the reference level v(k) is decreased. At the beginningof the transmission, the leak signal h(t) is large compared with theremote signal (b =+1, a=+1 or b=-1, a=-1) and the reference level shallbe increased simultaneously as c is increased, resulting in thatr(t)-y(t)+w(t) decreases. The balance filter then converges, accordingas the coefficient c increases and v(k) increases (the error signale.sub. k becomes less and less). When the balance filter has converged,the received signal r(t)=h(t) - y(t)+w(t) is very close to the remotesignal, except for a residue error which is dependent on the magnitudeof the stepping length of the parameters c_(k) in the balance filter.The effect on the balance filter convergence being dependent on theremote signal has thereby been cancelled, and thereby also theabove-mentioned problem with convergence of the balance filter.

An apparatus which performs the proposed method will now be describedwith reference to FIG. 8. In this Figure the transmitter unit S, hybridcoupler G and lowpass filter LP have been excluded. The signal w(t)+h(t)is sent from the lowpass filter to the summing circuit A1, the otherinput of which receives a signal y(t) from the output of adigital-to-analogue converter DA1. The output signal r(t)=w(t)+h(t) -y(t) is sampled in the circuit SH and gives a digital signal r(k)(k=1,2, . . . ,N) at the sampling instants (sampling interval T) andwith varying amplitude to the input of a quantizer Q. In the simple caseillustrated this comprises an operational amplifier Q working as acomparator, its plus input being connected to the sampling circuit SHand its minus input being connected to the reference potential 0(ground). The comparator Q thus decides whether the sampled signal'slevels are greater or less than zero. Such an implementation of aquantizer can be used if only two levels of the incoming signalw(t)+h(t) are to be detected. The output magnitude a_(k) from thecomparator Q is thus still 0 or 1.

The apparatus in accordance with the invention further includes: thereference unit V, having a memory unit MV, a digital-to-analogueconverter DA2 connected to the output of the memory unit MV and acorrelator KV including logic circuit E3 and a summing circuit A4 andtwo multipliers M3 and M4. A reference value v(k) is stored in thememory unit MV and is utilized in both comparators J1 and J2, afterconversion to its analogue value in the converter DA2. The value v(k)can be updated in the memory unit MV via a write input connected to theoutput of a summing circuit A4. One input of the summing circuit A4 isconnected to the-output of the memory unit MV and the other to theoutput of the multiplier M4. The multiplier M3, connected to one inputof the multiplier M4, forms a value β· vk which, in certain logicalconditions, occurs on the output of the multiplier M4 and thereby givesa new value (β+1)v(k) for writing into the memory unit MV.

The comparator units J1 and J2 are each connected with their minus andplus inputs to the output of the digital-to-analogue converter DA2, andwith their plus and minus inputs connected to the output of the samplingcircuit SH, across which the quantity r(k) occurs. An inverter circuitI1 is connected between the plus input of the comparator J2 and theconverter DA2. The outputs of the comparators Q and J2 are connected toan AND circuit O1 with inverting inputs, the output of which isconnected to one input of an OR circuit E1, the second input of which isconnected to the comparator J1. The comparators J1 and J2 thus comparethe sample values r(k) with the analogue value +v(k) and -v(k),respectively, i.e. decide whether the sample values r(k) are above apositive reference level +v(k) or under a negative reference level-v(k). The logic O1, E1 subsequently takes on a state corresponding tothe different cases which may occur (this is discussed in detail below).

To the outputs of the OR circuit E1 and the comparator Q there isconnected a sign-forming circuit which comprises two AND gates O2 andO3, an exclusive-OR circuit E2 and three controllable switches K1-K3 forsending a +1, 0 or -1 state in response to the binary states at therespective gate outputs. The outputs of the switches are mutuallyconnected to form the output of the sign-forming circuit. This output isconnected to one input of a first multiplier M1, the second input ofwhich is connected to a second multiplier M2. The value v(k) from thememory unit MV in the reference unit occurs at one input of themultiplier M2, and at the other input a constant value α (in digitalform). In the case where the balance filter B is a conventional FIRfilter containing delaying links and multipliers, the outputs of theunits K1-K3 are connected to one input of a multiplier M5, its otherinput receiving the data flow b_(k) (cf FIG. 2). In this case there ishowever a memory filter which is described below. A summing circuit A5is connected by one input to the output of the multiplier M1 and by itsother input it is connected to the output of a coefficient memory CM inthe balance filter B. This may be an FIR filter, which is a known kindof memory filter. The filter B contains a shift register SB with Npositions, an address register AE to form an address from the N valuesin the positions, and a coefficient memory CM. The shift register SBstores the values b_(k), b_(k-1), . . . b_(k-N+1). These values give aninteger presentation a(b_(k)). An address j =a(b_(k)) is formed in theaddress register AE, to point out a given coefficient c_(j) (k) in thecoefficient memory CM. An array of digital values c_(j) is obtainedacross the output of the balance filter, these values being the signaly(t) via the converter DA1. The output of the summing circuit A5 isconnected to an input of the memory CM for updating the coefficientsc_(j) (k) to c_(j) (k+1). The signal y(t) is added to the incominganalogue signal w(t) from the lowpass filter to form the signal r(t).

The operation of the apparatus will now be described in detail withreference to the time diagram according to FIG. 3. The binary data flow(b_(n)) transmitted from a data source (telephone exchange) to thetransmitter unit S is illustrated in FIG. 3. The transmitter may includea conventional coding unit, which may convert the data flow b_(n) tobiphase code and a corresponding analogue biphase coded signal s(t) isshown in FIG. 3. As previously described, a part of the signal s(t) willpass through the coupler G and be added as the signal h(t) to theanalogue biphase coded signal w(t) across the line L. After sampling thesignal r(t)=w(t)+h(t) - y(t) the sample values r(k) are obtained. Thecompartors Q and J1, J2 in FIG. 8 now decide whether these sample valuesare greater or less than zero and if their absolute values are greaterthan or less than the reference level v(k). If it is assumed that eachof the comparators Q and J1, J2 give 1 if the level at the associatedplus input is greater than the level over the minus input, and 0 if thelevel is less the following truth can be given:

    __________________________________________________________________________             I      II     III     IV                                             __________________________________________________________________________    Input,   r(k)>0 and                                                                           r(k)>0 and                                                                           r(k)<0 and                                                                            r(k)<0 and                                     quantizer Q:                                                                           r(k)>ν(k)                                                                         r(k)<ν(k)                                                                         /r(k)/>/ν(k)/                                                                      /r(k)/</ν(k)                                Output (a.sub.k)                                                                       1      1      0       0                                              comparator Q:                                                                 Output,  1      0      0       1                                              OR gate E1:                                                                   Output ε.sub.k =                                                               +1     0      -1      0                                              sign e.sub.k:                                                                 Output, summing                                                                        α · ν(k) · b.sub.k                                        c.sub.j (k)                                                                          c.sub.j (k)-                                                                          c.sub.j (k)                                    circuit A5:                                                                            +c.sub.j (k)  -α · ν(k) · b.sub.k         Output, summing                                                                        (β + 1)ν(k)                                                                  v(k)   (β + 1)ν(k)                                                                   ν(k)                                        circuit A4:                                                                   Output, E3:                                                                            1      0      1       0                                              __________________________________________________________________________

In cases I and III in the table above, the signal value r(k) is greaterthan the reference level v(k), resulting in that it will be increased bya factor (1 + β)v(k). Furthermore, the filter parameters c_(j) will becorrected by a factor α· v(k).

In case I, this factor will be positive, i.e. the parameters will beincreased if b_(k) =1 and negative if b_(k) = -1, and in case IIInegative if b_(k) = 1 and positive if b_(k) = -1.

In cases II and IV, the signal value r(k) is less than the referencelevel v(k). This will then be decreased by the factor (1- β)v(k). On theother hand, the filter parameters c_(j) are not corrected, since ε_(k) =0.

A residue error remains after convergence of the balance filter, thesize of this error being in proportion to the size of the step lengthsused in updating the parameters c_(j) (k). For higher received r(k)signal levels, a larger residue error than for lower ones may betolerated. Since the reference value v(k) is a measure of the signalstrength of r(k), the convergence may be further accelerated if the steplength is controlled by the reference value. In FIG. 1, a dashedconnection has been denoted between the units V and Q, i.e. quantizingof the incoming sample values r(k) is dependent on the reference levelv(k). At the start of the adjustment, r(k) is typically dominated by thecontribution from the local transmittter S, i.e. h(k). The referencevalue v(k) then adapts itself to a level determined by the sample h(k)and the adaption is carried out with great step length. By degrees, asthe convergence continues, r(k) diminishes and thereby the referencevalue v(k) until the balance filter B is completely converged, with theexception of an acceptable residue error.

What we claim is:
 1. In a telecommunication system wherein digitalinformation is transmitted in duplex via a hybrid circuit and a singleconductor pair between first and second transmitter-receiver means, themethod of minimizing at said first transmitter-receiver meansdisturbances in a line signal received via said single conductor pairand said hybrid circuit from said second transmitter receiver means whensaid first transmitter-receiver means is also transmitting signalsrepresenting data symbols b_(k) to said second transmitter receivermeans, said method comprising the steps of providing in said firsttransmitter-receiver means a balancing filter means for creating aplurality of balancing signals, subtracting a selected balancing signalfrom the line signal received from said second transmitter-receivermeans and the disturbances arising from the signal being transmitted bysaid first transmitter-receiver means to provide a difference signal,periodically sampling the difference signal to generate a sampledsignal, quantizing said sampled signals to form an estimation of thedigital data represented by said line signal, generating a controllablyvariable reference signal, multiplying said reference signal by saidestimation signal to form a product signal, substracting said sampledsignal from said product signal to form an error signal correlating saiderror signal and said sampled signal, varying said controllably variablereference signal in response to said correlating step to decrease saiderror signal, adding said error signal to said sampled signal to form anauxiliary signal, correlating said auxiliary signal with a data symbolthen represented by a transmitted signal and changing the selectedbalancing signal created by said balancing filter so that there is lesscorrelation between said auxiliary signal and said data symbols wherebythe influence on the convergence of said balance filter by said linesignal is eliminated.
 2. In a telecommunication system wherein digitalinformation is transmitted in duplex via a hybrid circuit and a singleconductor pair as a duplex path between first and transmitter-receivermeans apparatus for minimizing at said first transmitter-receiver meansdisturbances in a line signal received via said single conductor pairand said hybrid circuit from said second transmitter-receiver means whensaid first transmitter-receiver means is also transmitting signalsrepresenting data symbols, said apparatus comprising a balance filtermeans including balancing signal generating means for generating aselected one of a plurality of balancing signals, first subtractingmeans for subtracting the balancing signal from the line signal receivedfrom said second transmitter-receiver means and the disturbances arisingfrom the signal being transmitted by said first transmitter-receivermeans to provide a difference signal, sampling means for periodicallysampling said difference signal to generate a sampled signal, quantizingmeans for quantizing the sampled signal to form an estimation signal,controllably variable reference signal means for generating a referencesignal, first multiplying means for multiplying the reference signal bythe estimation signal to form a product signal, second subtracting meansfor subtracting the product signal from the sampled signal to provide anerror signal, first correlating means for correlating the error signaland the sampled signal to generate a control signal to control saidcontrollably variable reference signal means to vary the generatedreference signal in such a manner to decrease the error signal, secondcorrelation means for correlating the sum of the error signal and thesampled signal with the data symbol then represented by a transmittedsignal to control said balance filter means to select such otherbalancing signal to lessen the correlation between the error signal andthe sampled signal.